Phenology of Avicennia Marina (Forsk.) Vierh. in a Disjunctly-Zoned Mangrove Stand in Kenya

Phenology of Avicennia marina (Forsk.) Vierh. in a Disjunctly-zoned Mangrove Stand in Kenya

V. W. Wang’ondu 1,2, 3, J. G. Kairo2, J. I. Kinyamario1, F. B. Mwaura1, J. O. Bosire2, F. Dahdouh-Guebas3,4 and N. Koedam3

1University of Nairobi, School of Biological Sciences, P. O. Box 30197, Nairobi, Kenya; 2Kenya Marine and Fisheries Research Institute (KMFRI), P. O. Box 81651, Mombasa, Kenya; 3Biocomplexity Research Focus c/o Laboratory of General Botany and Nature Management, Mangrove Management Group, Faculty of Sciences, Vrije Universiteit Brussel - VUB, Pleinlaan 2, B-1050 Brussel, Belgium; 4Biocomplexity Research Focus (Complexité et Dynamique des Systèmes Tropicaux), Département de Biologie des Organismes, Faculté des Sciences, Université Libre de Bruxelles - ULB, Campus du Solbosch, CP 169, Avenue Franklin D. Roosevelt 50, B-1050 Bruxelles, Belgium.

Keywords: Avicennia marina, phenology, leaf emergence, leaf fall, fruiting, landward, seaward.

Abstract—Avicennia marina in Gazi Bay, Kenya, displays a disjunct zonation pattern across the intertidal zone with a seaward and a landward A. marina fringe. Earlier studies revealed significant differences in its vegetation structure, physiognomy, root system and leaf morphology, which can be attributed to salinity and tidal inundation differences that characterise the forest zones. The main objective of this study was to investigate the phenology of A. marina in the disjunctly zoned stands by direct shoot observation. Vegetative and reproductive phenology of A. marina was studied from January 2005 to December 2006. Four natural and one reforested sites were used for the study in the landward and the seaward intertidal zone. Randomly selected shoots (54 per site) were carefully tagged for direct shoot observation and sampling done every fortnight for leaf emergence and fall, and bud, flower and fruit production. Vegetative and reproductive attributes of the species were clearly seasonal in both zones with distinct patterns. However, shifts in peaks in leaf fall and emergence were observed in 2006. Unimodal and bimodal leaf fall patterns were respectively observed at the landward and seaward sites. Monthly leaf emergence and fall was significantly different (p<0.05) within sites, but not significantly different (p>0.05) between sites. Mean leaf longevity was 11 months with a significant difference (p<0.05) between the seaward reforested site and the landward site. Bud initiation occurred in November in both zones. However, flowering occurred earlier and the fruiting period was shorter in the landward zone compared to late flowering and prolonged fruiting in the seaward zone. Fruit fall peaked in April and May during the wet season. Differences in the vegetative and reproductive phenology of A. marina across the intertidal zone are discussed.

Corresponding Author: VWW E-mail: [email protected] 136 V. W. WANG’ONDU ET AL

INTRODUCTION studies conducted at Gazi Bay in Kenya from 1993 to 1994 concentrated on the landward Avicennia marina is a mangrove tree species stands of A. marina (Ochieng & Erftemeijer, that is extraordinarily adaptable with a wide 2002). However, data from this study can be latitudinal range closely associated with used to compare past and present phenological its flexible growth pattern. It is common characteristics of the species. The present study throughout the Indo-Pacific region within thus set out to investigate the phenology of A. a latitudinal range of 30o N to 38o S (Duke, marina in landward and seaward mangrove 1990). In Kenya, A. marina is the third most stands. The aim was to establish the existence dominant mangrove species after Rhizophora of similarities or differences in the phenological mucronata Lamk. and Ceriops tagal (Perr) C. attributes of A. marina in the two zones. B. Robinson (Kairo et al., 2002). The species displays a disjunct zonation pattern across MATERIALS AND METHODS the intertidal complex (Dahdouh-Guebas et Study area al., 2002). A comparison of landward and o seaward A. marina zones revealed significant The study was conducted at Gazi Bay (4 25’S, o differences in physiognomy (Dahdouh-Guebas 39 30’E) located 50 km south of Mombasa, et al., 2004, 2007). Avicennia marina trees in in the Kwale district (Fig.1). The total area of 2 the seaward zone are also structurally complex Gazi embayment is 18 km , with mangrove 2 in terms of plant height and stem diameter, forests occupying 6.6 km (Slim et al., 1996). while those in the landward zone are dwarfed. Extensive phenological studies of A. marina have been undertaken (Wium- Andersen & Christensen, 1978; Duke, 1990; Hegazy, 1998; Coupland et al., 2005). Wium- Andersen and Christensen (1978) observed that species growing in the landward part of a mangrove forest with large seasonal variations in groundwater salinity had a unimodal growth pattern, while species in more frequently flooded areas had a bimodal pattern. Coupland et al., (2005) observed a unimodal leaf growth pattern in A. marina trees growing in the highest intertidal area. Apart from studies by Steinke and Charles (1984) in South Africa and Ochieng and Erftemeijer (2002) in Kenya, there is little documentation on the phenology of mangroves in the Western Indian Ocean region, making a comparative study with other regions difficult. However, these were carried out by litter fall studies and not through direct shoot observation. In Australia, the phenology of at least 35 mangrove species has been documented (Duke et al., 1984; Duke, 2006). Documentation of the phenology Fig. 1: Map of the Kenyan Coast (top and of mangroves is important if the effects of bottom) showing study sites 1-3 at Gazi Bay, representing the landward A. marina sites, while climate change on these forests are to be well- sites 4 and 5 represent the seaward natural and understood, especially since mangrove forests reforested sites respectively. (Modified from are faced with the problem of sea level rise Dahdouh-Guebas et al., 2000; Dahdouh-Guebas due to climate change. Previous phenological et al., 2002; Neukermans et al., 2008.) Phenology of Avicennia marina (Forsk.) Vierh. in a Disjunctly-zoned Mangrove Stand in Kenya 137

The climate is influenced by the South-East Phenological observations Monsoon (April-October) and the North-East At each site, fifty four shoots from nine monsoon (November-March) winds, during randomly selected trees were marked for which the long rains (April-July) and short easy identification. The shoots were tagged rains (October–December) occur respectively. at an accessible height in the crown canopy The ten mangrove species found in Kenya for phenological shoot observations. Leaves occur at Gazi Bay, the dominant species present at the beginning of the study were being A. marina, C. tagal and R. mucronata numbered consecutively on the adaxial (Dahdouh-Guebas et al., 2002). Mangroves of surface with a xylene-free permanent marker. Gazi are heavily harvested for firewood and Care was taken not to damage the leaves. In building poles leading to their degradation. A subsequent sampling, unnumbered leaves were programme to rehabilitate degraded mangroves treated as newly-emerged and were numbered sites in Gazi was initiated in 1990 (Kairo, 1995; with further consecutive numbers. Numbered Kairo et al., 2001). leaves that were lost were recorded as leaf fall. Study design Newly-emerged and lost leaves were recorded Four natural sites and one reforested fortnightly from January 2005 to December monospecific A. marina stand were used for 2006. The shoots were carefully monitored the study. Three natural sites were located in for reproductive structures (buds, flowers the landward zone, whereas two sites (natural and fruits) until the mature propagules fell, and reforested) were located in the seaward providing a reproductive phenological record zone. The only reforested site used for the of the species in the respective sites. Data study was established in 1993 (Kairo, 1995). on litter fall were also collected but are not A randomly selected 10 m × 10 m plot was reported here. Climatic data (rainfall and air established in each of the five mangrove sites. temperature) were obtained from the Mombasa Meteorological Station; the precipitation in The D130 and height of the trees in the plot was measured to establish the stand characteristics 2005 and 2006 totalled 848 mm and 1581 mm respectively, with mean annual air temperatures of trees with a D130 of ≥2.5 cm. Basal area and stand densities were derived from these data. respectively of 28.1°C and 30°C (Fig. 2). Canopy cover was also estimated. Problematic tree architectures were dealt with as suggested by Dahdouh-Guebas and Koedam (2006).

2005 Rainfall 2006 Rainfall 2005 Temperature 2006 Temperature

300 34 250 32 30 200 28 150 26 100

Rainfall(mm) 24

50 22 Temperature(oC) 0 20 J F M A M J J A S O N D Months

Fig. 2: Monthly rainfall (mm) and mean monthly temperatures (°C) at Mombasa in 2005-2006. April- June comprises the long rain season and October-November the short rain season. (Source: Mombasa Meteorological Station.) 138 V. W. WANG’ONDU ET AL

Statistical analysis to December in both years (Fig. 4). Leaf fall was lower in 2006 than 2005 at all A. marina All data were analysed using STATISTICA sites, especially during the months of March 6.0 software. Monthly, annual and site means to August. were compared using One-way ANOVA and There was no significant difference in the Kruskal Wallis non-parametric test when leaf fall between the five sites (p>0.05) or the data did not meet parametric assumptions, in annual leaf loss within sites. However, even after log transformations. Tukey’s significant differences in monthly leaf fall Honestly Significant Difference (HSD) test were observed in the year 2006 (p<0.05) and post hoc multiple comparisons of means between the months of January to July and were carried out to establish differences August to October. Leaf fall peaks generally between means. coincided with the dry months of January- RESULTS February and July-September. However, 2006 was a wetter year and leaf fall patterns at the Stand characteristics seaward sites appeared to be disrupted. Structural and stand characteristics of A. Seasonality in leaf emergence marina in the two zones are presented in Table Leaf emergence in A. marina was also 1. Landward sites 1, 2 and 3 had shorter trees seasonal and displayed unimodal and bimodal compared to seaward sites 4 and 5, with trees patterns at the landward and seaward sites in site 1 bordering the Gazi village being taller respectively. At the former, leaf emergence than those at sites 2 and 3. The percentage peaked from March to September in 2005 and canopy cover was also lower in the landward September to November in 2006. The seaward than in the seaward sites, with site 1 having a sites manifested peaks in leaf flush in February higher canopy cover than sites 2 and 3. and from July to September in 2005. Leaf emergence at these sites was reduced in 2006, Vegetative phenology but major peaks occurred from September to Seasonality in leaf fall November. Little or no leaf emergence was observed during the months of January to Leaf fall in the landward A. marina zone was May at all sites in 2006. There was a close seasonal and characterised by a unimodal leaf relationship between leaf emergence and leaf fall pattern in the two years of study (Fig.3). fall, the latter occurring 1-2 months after peak In 2005, the major peak leaf fall occurred leaf emergence. There was no significant between the months of July to September at difference in leaf emergence among the these sites, whereas in 2006 leaf fall peaks sites (p>0.05). Monthly leaf emergence was, occurred in September to November. The however, significantly different at all sites seaward sites displayed a bimodal leaf fall (p<0.001). pattern, with peaks in February and September Table 1: Structural characteristics (means ±S.D) of A. marina at the five study sites in Gazi Bay. n = number of trees in each site with a D130 of ≥ 2.5cm. ns = natural site; rs = reforested site

Site n Mean height D130 Density Basal area Total cover ` (m) (stems/ha-1) (m2/ha-1) (%) 1 (Landward ns) 50 4.94±0.51 7.81±2.81 5 000 0.28 70-80 2 (landward ns) 71 3.71±0.68 4.4±1.47 7 100 0.13 40-50 3 (Landward ns) 63 2.73±0.52 6.28±1.99 6 300 0.22 40-50 4 (Seaward ns) 37 6.34±0.81 11.02±5.14 3 700 0.44 80-90 5 (Seaward rs) 41 5.1±0.6 7.11±3.65 4 100 0.22 60-70 Phenology of Avicennia marina (Forsk.) Vierh. in a Disjunctly-zoned Mangrove Stand in Kenya 139

Table 2: Mean, minimum and maximum longevity (months) of A. marina leaves at sites in Gazi Bay. Ns = natural site; rs = reforested site; n = number leaves that emerged and fell during the study period. Site description n Mean±(S.E) Minimum Maximum 1 (Landward ns) 278 11.22 (4.73) 0.93 18.66 2 (Landward ns) 292 10.56 (3.88) 1.40 16.80 3 (Landward ns) 320 10.95 (3.67) 0.46 17.73 4 (Seaward ns) 124 10.63 (4.07) 1.86 18.66 5 (Seaward rs) 233 9.24 (4.04) 0.93 16.80

Trends in reproductive phenology fruit production between the sites. Peak fruit production occurred in April and May which The timing of the reproduction cycle in coincided with the wet season. It is notable A. marina was seasonal and its initiation that reproductive activity in 2005 was lower coincided with the short rainy season. Bud at all sites compared to 2006 and, though bud initiation started in the month of November production was recorded at all the sites in 2005, at all A. marina sites during the study period. fruits were only found at the reforested site. Peaks in bud production were observed in December and January at all the sites (Fig. 3 Leaf longevity and 4). However, no new buds were produced after February at the landward sites, whereas Mean leaf longevity in A. marina was 11 bud formation was prolonged at the seaward months (Table 2). There was a significant sites until April and March in 2005 and 2006 difference in leaf longevity between the sites respectively (Fig.3). Bud production was (p<0.001), with the seaward reforested site much lower at all sites in 2005 than in 2006. manifesting the shortest leaf longevity of Bud production also differed significantly 9.24 ± 4.04 months and site 1 the greatest leaf among sites (p<0.001). Landward site 1 and longevity of 11.22 ± 4.73 months seaward site 4 differed significantly from landward sites 2 and 3 (p<0.01) as they DISCUSSION produced the least number of buds. At the landward sites, flower production Vegetative phenology commenced in November to March in site Seasonality was observed in A. marina leaf 2 and December to March in sites 1 and 3 production and fall at both the landward and (Fig. 3). It started later at the seaward sites, seaward sites but the period of leaf production from January to March in site 4, and January was longer at the former (January-April) to May in site 5 (Fig. 4). Flower production in 2005. Thus the vegetative phenology of was higher in 2006 than in 2005 but lower landward and seaward A. marina varied with than bud production at all the sites. There respect to the timing and abundance of leaf was a significant difference (p<0.001) in emergence and fall. flower production between sites, with site 4 Unimodal and bimodal peaks in mangrove producing the least flowers and having the leaf production are known to be associated with shortest flowering period of three months. forests growing in higher and lower intertidal Avicennia marina fruits were observed areas respectively due to fluctuations in ground from the months of March to May at the water salinity (Wium-Andersen, 1981). High landward sites with a peak in April. At the interstitial water salinity elevates stress in seaward sites, fruits were observed from mangroves, resulting in increased leaf loss to March to May in site 5 and July in site 4. There reduce water loss from transpiration, especially were no significant differences (p>0.05) in during the dry season, thus reducing water 140 V. W. WANG’ONDU ET AL (Site 1) Leaf fall Leaf emergence Buds Flow ers Fruits (Site 1) Leaf fall Leaf emergence Buds Flow ers Fruits (Site6 1) 0.35 (Site 1) Leaf fall Leaf emergence Buds Flow ers Fruits (Site 1) 6 Leaf fall Leaf emergence Buds Flow ers Fruits 0.35 0.30 5 6 0.35 6 0.350.30 65 0.350.25 0.30 4 5 0.300.25 54 0.30 0.250.20 4 0.250.20 3 4 0.25 43 0.200.15 0.200.15 2 3 0.20 32 0.150.10 Mean Leaf Mean fall & 0.150.10 Mean Leaf Mean fall & 2 0.15 1 2 0.10 Mean Leaf Mean fall & 21 0.05 emergence, Buds/Shoot emergence, 0.100.05 emergence, Buds/Shoot emergence, Mean Leaf Mean fall & 0.10 Mean Leaf Mean fall & Mean Flowers, Mean Fruits/Shoot 1 0.05 Flowers, Mean Fruits/Shoot 0 Buds/Shoot emergence, 10 0.050.000.00 emergence, Buds/Shoot emergence,

0.05 Flowers, Mean Fruits/Shoot emergence, Buds/Shoot emergence, J F M A M J J A S O N D J F M A M J J A S O N D Flowers, Mean Fruits/Shoot J0 F M A M J J A S O N D J F M A M J J A S O N D0.00 Flowers, Mean Fruits/Shoot 0 0.00 0 J F M A M J J A S O N D J F M A M J J A S O N D 0.00 J F M A 2005M J2005J A S O N D J F M A M J2006J A S O N D 2005 2006 2005 2006 (Site 2) (Site 2) (Site 2) (Site 2) 10 1.2 10 10 1.21.2 10 1.2 108 1.21.0 1.0 8 8 1.0 8 1.00.8 86 0.80.8 6 6 0.80.6 6 0.8 64 0.60.6 0.60.4 4 0.6

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2005 2006 Phenology of Avicennia marina (Forsk.) Vierh. in a Disjunctly-zoned Mangrove Stand in Kenya 141 (site(site 4) 4) LeafLeaf fall fall LeafLeaf emergence emergence BudsBuds Flowers Flowers FruitsFruits

(site 4) Leaf fall Leaf emergence Buds Flowers Fruits 66 0.080.08 55 0.060.06 446 0.08 335 0.040.04 0.06 24 Buds/Shoot 2 Buds/Shoot 0.020.02 113 0.04 02 0.00 Buds/Shoot Buds/Shoot 0 0.00 Mean Leaf Mean fall &emergence, Mean Flowers &Fruits/Shoot Flowers Mean Mean Leaf Mean fall &emergence, 0.02 &Fruits/Shoot Flowers Mean 1 JJ FF MM AA MM JJ JJ AA SS OO NN DD JJ FF MM AA MM JJ JJ AA SS OO NN DD 0 2005 2006 0.00 Mean Leaf Mean fall &emergence,

Mean Leaf Mean fall &emergence, 2005 2006 Mean Flowers &Fruits/Shoot Flowers Mean Mean Flowers &Fruits/Shoot Flowers Mean J F M A M J J A S O N D J F M A M J J A S O N D

2005 2006

(site(site 5) 5)

66 0.50.5 (site 5) 55 0.40.4 6 0.5 44 5 0.30.3 0.4 33 4 0.20.2 22 0.3 Mean Leaf fall & fall LeafMean Mean Leaf fall & fall LeafMean 3 1 0.10.1 emergence,Buds/Shoot 1 0.2 emergence,Buds/Shoot

2 &Fruits/Shoot Flowers Mean Mean Flowers &Fruits/Shoot Flowers Mean Mean Leaf fall & fall LeafMean Mean Leaf fall & fall LeafMean 00 0.00.0 1 0.1

emergence,Buds/Shoot 1 emergence,Buds/Shoot JJ FF MM AA MM JJ JJ AA SS OO NN DD JJ FF MM AA MM JJ JJ AA SS OO NN DD Mean Flowers &Fruits/Shoot Flowers Mean Mean Flowers &Fruits/Shoot Flowers Mean 0 20052005 20062006 0.0 Fig 4: Leaf fall andJ Femergence,M A M productionJ J A S ofO buds,N D flowersJ F M andA fruitsM J inJ A. AmarinaS O atN theD seaward Sites 4 and 5 in Gazi Bay. 2005 2006 stress. Increased leaf loss by the landward A. little vegetative activity while the trees were marina trees during the dry season is indicative flowering and fruiting, and leaf production of their adaptive measures to reduce water loss. and fall only resumed in June after completion The lack of clear unimodal and bimodal leaf fall of fruit fall. In 2005 there was little or no and emergence patterns at both the landward reproductive activity and it was associated and seaward sites in 2006 compared to 2005 with higher leaf fall and emergence. These may be attributable to the high and continuous results are corroborated by other studies rainfall experienced in that year. This may have (Duke et al., 1984; Wium-Andersen and lowered the salinity levels of the interstitial Christensen, 1978; Coupland et al., 2005). water, thus reducing salt stress and changing Increased leaf production by A. marina the leaf fall and emergence patterns. has been reported during the wet season in The lower leaf fall and emergence in 2006 Darwin Harbour in Australia (Coupland et may also be related to the reproductive status al., 2005). Ochieng and Erftemeijer (2002) of the trees at that time. There possibly was reported unimodal leaf flush patterns in a link between vegetative production and landward A. marina at Gazi in June to July (wet the reproductive cycle in that there was very season) in 1993-1994. In the present study, 142 V. W. WANG’ONDU ET AL similar observations were made between dry months which corroborates the results of July and September, which is normally a dry other studies (Duke et al., 1984: Fernandes season, indicating the likelihood of a shift in 1999; Ochieng & Erftemeijer, 2002). phenological trends in species with respect to Avicennia marina in Gazi Bay (4° 25’S, 39° the prevailing climatic conditions. Peak leaf 3’ E) was nevertheless observed to flower fall occurred one to two months after peak later than in Darwin Harbour (12° 26’S, 130° leaf emergence at all the sites, corresponding 51’ E) (Coupland et al., 2005). with findings onA. marina in Australia (Duke, Significant differences in bud and flower 1988). Shifting of leaf fall and emergence production between the sites can be related peaks to later months, especially in 2006, to stand structure and tree height. Avicennia indicates that the phenological patterns are marina trees at sites 2 and 3 were short and not static and may be influenced by changing stunted and had a lower canopy cover, thus climatic factors; this is evidenced by the allowing more light penetration due to less different rainfall and air temperatures during shading. Sites 1, 4 and 5 conversely had taller the two years of study. However, long-term trees and higher canopy cover. This may phenological studies are necessary to draw explain why a greater production of buds and conclusions regarding the effects of climate flowers was recorded at sites 2 and 3. change on mangrove phenology. Timing of fruit production in the two zones The leaf longevity in A. marina observed was uniform; however, fruits at the seaward in this study corresponded with that reported site 4 persisted up to June. Fruit fall in the two for the same species at Gazi Bay (Ochieng & zones coincided with the wet season, consistent Erftemeijer, 2002). It is close to the 14 months with findings in other studies (Duke et al., reported for the same species in Australia 1984; Fernandes, 1999; Ochieng & Erftemeijer, (Coupland et al., 2005). Significant differences 2002; Coupland et al., 2005). The reproductive in leaf longevities at the landward and seaward cycle of A. marina from bud initiation to fruit sites may be indicative of a response to fall was observed to last 6-7 months in the stress brought about by differences in ground landward zone and 7-8 months in the seaward water salinity resulting from differences in zone. Other studies have reported varying inundation. The seaward reforested and natural reproductive cycles at different latitudes, e.g. sites in this study manifested reduced leaf five months in Darwin Harbour (Coupland et lifespan. Frequent inundation of the seaward al., 2005), six months in southern Thailand sites results in less stress, hence possibly a (Wium-Andersen & Christensen, 1978) and higher leaf turnover. The seaward reforested one year in south-eastern Australia (Clarke & site 4 comprised a young stand (12 yrs old) Myerscough, 1991). Zonation may have had an that may have been actively growing, causing influence in fruiting ofA. marina since sites that a similar reduction in leaf longevity. were frequently inundated (seaward) supported prolonged budding and fruiting relative to Reproductive phenology those that were less inundated (landward sites). The synchronised initiation of budding at all The relationship between vegetative and sites may be indicative of the adaptability A. reproductive cycles cannot be ignored in this marina in the two zones. However, earlier study as reproductive months were associated flowering of trees in the landward zone than with very little vegetative activity, both in terms those in the seaward zones was indicative of of leaf production and fall. Leaf production a lack of uniformity in phenological trends peaked prior to bud initiation and again later in this species, even within the same locality. after fruit fall in May. This corroborates the Prolonged flowering at the seaward sites work of Duke (1990) who found a link in the may have resulted from more favourable timing of leaf appearance and leaf fall with environmental factors compared to the inflorescence development. This has been landward sites. Flowering peaked during the attributed to resource partitioning within plants Phenology of Avicennia marina (Forsk.) Vierh. in a Disjunctly-zoned Mangrove Stand in Kenya 143

(Duke et al., 1984), as evident in S. alba and Dahdouh-Guebas F, Mathenge C, Kairo A. marina (Coupland et al., 2005) as well as J, Koedam N (2000) Utilization of C. tagal, Lumnitzera littorea and A. marina mangrove wood products around Mida (Wium-Andersen & Christensen, 1978). Creek (Kenya) amongst subsistence and This study has thus shown that the vegetative commercial users. Economic Botany and reproductive phenology of A. marina at 54: 513-527 Gazi Bay is clearly seasonal, evidenced by Dahdouh-Guebas F, Verneirt M, Cannicci peaks in the various phenophases. However, S, Kairo J, Tack F, Koedam N (2002) vegetative peaks appeared not to be restricted to An explorative study on grapsid certain months since there were some shifts in crab zonation in Kenyan mangroves. peaks in separate years, suggesting a response Wetlands Ecology and Management 10: triggered by environmental factors. 179–187 Earlier studies indicated significant differences in vegetation structure, Dahdouh-Guebas F, De Bondt R, Abeysinghe physiognomy, root development and leaf D, Kairo J, Cannicci S, Triest L, morphology in trees in different zones which Koedam N (2004) Comparative study of could be attributed to differences in salinity and the disjunct zonation pattern of the grey the frequency of tidal inundation (Dahdouh- mangrove Avicennia marina (Forsk.) Guebas et al., 2004, 2007). This study Vierh. in Gazi Bay (Kenya). Bulletin of revealed differences in A. marina leaf growth Marine Science 74: 237–252 patterns between the landward and seaward Dahdouh-Guebas F, Koedam N (2006) zones studied. The timing and duration of the Empirical estimate of the reliability of the reproductive phenology of A. marina also use of the Point-centred Quarter Method varied in the two zones. This may be indicative (PCQM): solutions to ambiguous of variations and adaptive responses in this field situations and description of the species to prevailing environmental conditions PCQM+ protocol. Forest Ecology and in these zones. Management 228: 1-18 Acknowledgments–We gratefully thank Dahdouh-Guebas F, Kairo J, De Bondt R, Loise Kanyi and Ali Ahmed Yusuf for field Koedam N (2007) Pneumatophore assistance and data collection. We appreciate height and density in relation to micro- the support of the Vlaamse Interuniversitaire topography in the grey mangrove Raad (VLIR) programme secretariat during Avicennia marina. Belgian Journal of the course of this work. The lead author was Botany 140: 213-221 a doctoral student under the VLIR-IUC-UON programme of the University of Nairobi and Duke C (1988) Phenologies and litter fall of the Free University of Brussels. two mangrove trees, Sonneratia alba Sm. and S. caseolaris (L.) Engl., and their putative hybrid, S. x gulngai N.C. REFERENCES Duke. Australian Journal of Botany 36: Clarke J, Myerscough J (1991) Floral 473-482 biology and reproductive phenology Duke C (1990) Phenological trends with of Avicennia marina in South-Eastern latitude in the mangrove tree Avicennia Australia. Australian Journal of Botany marina. Journal of Ecology 78, 113-133 39: 283-293 Duke C, Bunt S, Williams T (1984) Coupland T, Paling I, McGuinness A (2005) Observation on the floral phenologies Vegetative and reproductive phenologies of Northeastern Australian mangroves. of four mangrove species from northern Australian Journal of Botany 32: 87-99 Australia, Australian Journal of Botany 53: 109-117 144 V. W. WANG’ONDU ET AL

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